This invention relates to means for recovering the modulation from a modulated signal and more particularly to a coherent demodulator which recovers phase modulation from a phase modulated signal and to means for stabilizing the coherent demodulator to permit its use.
Modern automatic radio direction finders (ADF) for aircraft use an antenna which is fixed to the skin of the aircraft and which includes what are known as loop and sense elements. In this description the word "loop" is used to describe different elements in various arts. Specifically, in the ADF art a loop element or antenna is a type of radiated signal response circuit which provides a portion of the signal input into the ADF receiver. In the phase locked loop art a loop is the closed circuit which includes usually at least a voltage controlled oscillator and a phase comparator. In the description to follow the two usages of the term "loop" are distinguished by use of the expression "loop element" or "loop antenna" when referring to the ADF art element and use of the term "loop" without the aforementioned qualifiers to refer to the phase locked loop art circuit. Continuing the discussion of the background, the loop element comprises two mutually perpendicular ferrite rods and electrical windings thereon. The amplitudes of the signals induced in the various windings by an electromagnetic field of the type radiated by a radio broadcast station is dependent upon the orientation of the loop elements with respect to the broadcast station. By considering the amplitudes of the induced signals the direction of the broadcast station from the loop elements can be ascertained with a 180.degree. ambiguity. The ADF antenna thus also includes an omnidirectional sense antenna which provides phase information to resolve the ambiguity.
Since the ADF antenna is located on the skin of the aircraft and the ADF receiver and other processing circuits are located in the pilot's cabin or cockpit, it is necessary to provide some convenient means for carrying the antenna signals to the receiver. This is accomplished by mixing the loop antenna signals individually with a low frequency square wave and modulating the sense antenna signals therewith. The resulting phase modulated signal contains all the radio direction information and it is conveyed to the ADF receiver through a single cable. It now remains to demodulate the signal at the receiver to obtain the direction information in a format which can be used to drive the ADF display. Demodulation is accomplished in a coherent demodulator comprising a phase locked loop having the modulated receiver intermediate frequency as an input. A reference signal generated in the phase locked loop is frequency compared against the receiver IF during capture and subsequently phase compared against receiver IF during locked operation of the phase locked loop. The phase locked loop error signal comprises the recovered phase modulation. Long time constant filters are used in the phase locked loop to prevent the rapidly varying phase modulation from affecting the loop generated frequency signal. A stability control loop which compares the phase locked loop frequency with the same frequency as passed through a narrow pass filter helps maintain the proper loop frequency and permits use of the coherent demodulator. The demodulated signals can be further processed to produce d.c. signals which can be applied to orthogonal windings on an ADF indicator to provide an ADF display.
As known to those skilled in the art airline ADF's normally comprise receivers capable of selecting stations separated by 500 Hz. However, general aviation ADF receivers seldom provide such selectivity. Instead, the general aviation pilot normally selects an ADF frequency 500 Hz above or below the assigned frequency. For example, for an assigned station frequency of 245.5 KHz the pilot's ADF control head will read 245 or 246 KHz. For conventional ADF receivers, i.e., those not having a coherent demodulator, the degradation from 500 Hz mistuning is acceptable, thus general aviation ADF's are less complex than airline ADF's. However, when a coherent demodulator is used the acquisition bandwidth is increased by 500 Hz plus station tolerance. In the system to be described the coherent demodulator can acquire lock to a carrier within .+-.770 Hz of the selected ADF frequency. This is adequate to cover receiver, ground station and VCO environmental drift. The locked bandwidth of the coherent demodulator to be described is approximately 6 Hz with a damping factor of 0.6. Thus, since the coherent demodulator loop bandwidth can only, practically, be reduced to about 6 Hz, while the demodulated information is about 31 Hz, it becomes important to control loop environmental drifts that might otherwise cause bearing errors.
It is thus an object of this invention to provide radio automatic direction finding equipment which provides a display of bearing from the equipment to a radio station.
Another object of the invention is to provide a stabilized coherent demodulator of an ADF receiver.
These and other objects of the invention will become apparent from a reading and understanding of the following description of one embodiment of the invention.